﻿// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

namespace System.Runtime.CompilerServices;

/// <summary>
/// Indicates that the use of <see cref="System.ValueTuple"/> on a member is meant to be treated as a tuple with element names.
/// </summary>
[AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue | AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Event)]
public sealed class TupleElementNamesAttribute : Attribute
{
    private readonly string[] _transformNames;

    /// <summary>
    /// Initializes a new instance of the <see
    /// cref="TupleElementNamesAttribute"/> class.
    /// </summary>
    /// <param name="transformNames">
    /// Specifies, in a pre-order depth-first traversal of a type's
    /// construction, which <see cref="System.ValueType"/> occurrences are
    /// meant to carry element names.
    /// </param>
    /// <remarks>
    /// This constructor is meant to be used on types that contain an
    /// instantiation of <see cref="System.ValueType"/> that contains
    /// element names.  For instance, if <c>C</c> is a generic type with
    /// two type parameters, then a use of the constructed type <c>C{<see
    /// cref="System.ValueTuple{T1, T2}"/>, <see
    /// cref="System.ValueTuple{T1, T2, T3}"/></c> might be intended to
    /// treat the first type argument as a tuple with element names and the
    /// second as a tuple without element names. In which case, the
    /// appropriate attribute specification should use a
    /// <c>transformNames</c> value of <c>{ "name1", "name2", null, null,
    /// null }</c>.
    /// </remarks>
    public TupleElementNamesAttribute(string[] transformNames)
    {
        _transformNames = transformNames ?? throw new ArgumentNullException(nameof(transformNames));
    }

    /// <summary>
    /// Specifies, in a pre-order depth-first traversal of a type's
    /// construction, which <see cref="System.ValueTuple"/> elements are
    /// meant to carry element names.
    /// </summary>
    public IList<string> TransformNames => _transformNames;
}